The Cascadia Region Earthquake Working group (CREW) prepared a very thorough and thoughtful analysis of the Cascadia Subduction Zone and the earthquake that’s coming.
An excerpt from their report follows…
Cascadia Subduction Zone
Lying mostly offshore, the CSZ plate interface is a giant fault— approximately 700 miles long (1,130 km). Here, the set of tectonic plates to the west is sliding (subducting) beneath the North American Plate. The movement of these plates is neither constant nor smooth: the plates are stuck, and the stress will build up until the fault suddenly breaks. This last happened in January 1700. The result was an earthquake on the order of magnitude 9.0, followed within minutes by a large tsunami—much like the earthquake and tsunami that struck Japan on March 11, 2011. Stresses have now been building along the Cascadia subduction zone for more than 300 years, and the communities of Cascadia can be certain that another great quake will again shake the region.
The last Cascadia Subduction Zone event in 1700 offers no written eye-witness accounts, although a few Native American and First Nations oral stories do relate some of the effects. Instead, scientists found the record of Cascadia’s past activity in the landscape itself, which was altered suddenly and in characteristic ways by these great earthquakes and the tsunamis they triggered. Once scientists discovered what to look for, they found evidence up and down the coastline, on land and on the seafloor, from British Columbia to California.
The world’s largest quakes occur along subduction zones. Dubbed great earthquakes, the magnitude of these events ranges from 8.0 to 9.0+ (the largest on record was a magnitude 9.5 quake off the coast of Chile in 1960). Their characteristics include prolonged ground shaking, large tsunamis, and numerous aftershocks. Because the magnitude scale is logarithmic, each increase of one unit signifies that the waves radiated by the earthquake are 10-times larger and 32-times more energetic. This means that a M9.0 quake releases 1,995 times more energy than a M6.8. The Great Indonesia earthquake and tsunami of 2004 that killed 250,000 persons, and the East Japan earthquake and tsunami of 2011 that killed 16,000 are recent examples of great subduction zone earthquakes.
Anatomy of a Cascadia Zone Earthquake
The Cascadia Subduction Zone stretches from Cape Mendocino in northern California to Brooks Peninsula on Vancouver Island in British Columbia, a distance of about 700 miles (1,130 km). All along this zone, which begins beneath the seafloor to the west and extends inland towards the Cascade and Coastal mountains, the subducting plates are forced beneath the North American Plate. At a relatively shallow depth (less than about 20 miles/30 km down), the plates have become stuck. Below this locked zone, warmer temperatures make the plates more pliable, allowing them to move more readily past each other. This freer movement deeper down causes strain to accumulate along the locked zone. Once that strain is great enough to overcome the friction that keeps the plates locked, the fault will rupture: the edge of the North American Plate will lurch suddenly upwards and southwestwards as the subducting plates slip under and northeastwards. With this movement, the deformed western edge of the North American Plate will flex, causing the land along large sections of Cascadia’s coastline to drop as much as 6.6 feet (2 m) in elevation—an effect known as co-seismic subsidence.
Although it is possible that the Cascadia subduction zone will rupture section by section in a series of large earthquakes (each measuring magnitude 8.0 to 8.5) over a period of years, the earthquake that many scientists and emergency planners anticipate is modeled on the zone’s last major quake. The entire fault zone ruptures from end to end, causing one great earthquake measuring magnitude 9.0. The shaking that results from this abrupt shifting of the earth’s crust will be felt throughout the Pacific Northwest—and the ground is expected to go on shaking for four to six minutes.
Magnitude is a measure of an earthquake’s size: it tells how much energy is released when a fault ruptures. For the people and structures experiencing the earthquake, the intensity of the shaking is what really matters. In general, the intensity and destructiveness of the shaking will be greater the closer one is to the plate interface, with coastal areas experiencing the highest intensities and the level of shaking diminishing the farther inland one goes.
How much the ground shakes, or the shaking intensity, depends on one’s location. Proximity is a major factor (the closer you are to the rupture, the more intense the shaking tends to be), but the shape and consistency of the ground makes a big difference. In the 2001 Nisqually earthquake, the greatest shaking intensities were not nearest the rupture, but in areas where the soft soils of river valleys and artificial fill amplified seismic waves, such as on Harbor Island in Seattle.
Earthquakes cause damage by strong ground shaking and by the secondary effects of ground failures and tsunamis. When the Cascadia Subduction Zone ruptures, it will cause part of the seafloor to move abruptly upward. This displaces the column of water above the rupture. The result is a tsunami – a series of waves that travel outward in all directions from the place where the uplift occurred. Unlike wind-generated waves that travel along the surface, tsunami waves move through the entire body of water from seafloor to surface. Tsunami waves have extremely long wavelengths and contain a much greater volume of water than surface waves. This means that they look and act less like an ordinary wave and more like a vast, moving plateau of water.
A tsunami can travel across the deep ocean at nearly 500 miles (800 km) per hour. In deep water, the amplitude or height of the tsunami is low relative to its length, so the slope of the waves is very low, and they may pass unnoticed under ships. Upon entering shallower water, however, they slow down and gain in height as water piles up behind the wave front. Once it hits shore, a single tsunami wave can take as much as an hour to finish flowing in. The height of the wave and how far inland it travels vary with location: In places along Cascadia’s coast, the tsunami may be as high as 30 to 40 feet (9 to 12 m). Much depends on the local topography—the lay of the land—both underwater and along the shore. In general, the inundation will be greater where the land is low or where the topography focuses the waves, such as at bays and river mouths. Other key factors are subsidence and tides: when the fault ruptures, the land in many coastal areas will drop in elevation, increasing the run-up of the subsequent tsunami; and if the quake occurs during high tide, the tsunami will travel farther inland than it would at low tide.
Because the Cascadia Subduction Zone is close to shore, the first wave will reach land soon after the earthquake— within 20 to 30 minutes in some areas. Coastal residents can then expect to witness multiple waves over a period of hours. In addition, because parts of the coastline will have dropped (subsided) during the earthquake, some areas may remain flooded, or may continue to flood during high tide, even after the tsunami retreats.
The Cascadia earthquake is likely to be followed by aftershocks, which will occur throughout the region and vary in size. After a main shock as large as magnitude 9.0, a few aftershocks are likely to exceed magnitude 7.0. During the first month after the magnitude 8.8 Maule earthquake in 2010, Chile experienced 19 aftershocks larger than magnitude 6.0 (the largest was magnitude 6.9). Japan’s magnitude 9.0 Tohoku earthquake in 2011 was preceded by a magnitude 7.5 foreshock and followed by multiple aftershocks, the largest of which measured magnitude 7.9. Some of these aftershocks occurred on the west side of Honshu, demonstrating that such quakes may be triggered some distance from the main shock.
Aftershocks that follow hard on the heels of the main shock can bring down already weakened buildings. While the size and frequency of aftershocks will diminish over time, a few may cause additional damage long after the initial quake. This occurred in New Zealand, where the magnitude 7.0 Darfield earthquake in September of 2010 was followed by a magnitude 6.1 aftershock over five months later, which caused far more damage to the city of Christchurch than the main shock.
Landslides and Liquefaction
Local geologic conditions, including soil type, can increase or decrease the intensity of the shaking and produce a range of secondary effects, including landslides, liquefaction, and lateral spreads.
Liquefaction is one of the most damaging effects of ground shaking. Certain soils, such as water-saturated silt and sand, can become dangerously unstable during an earthquake. The shaking increases water pressure, forcing the water to move in between the individual grains of soil; as the grains lose contact with each other, the soil begins to act like a liquid. Overlying layers of sediment can slump and spread laterally. Structures built on such soils may shift position or sink, while buried pipes and tanks become buoyant and float to the surface. Liquefaction-prone soils are common in river valleys, along waterfronts, and in places covered with artificial fill. Unfortunately, these sites are often prime locations for important structures, including bridges, ports, airports, and industrial facilities. Many of the region’s most densely populated areas — such as along the I-5 corridor between Eugene and Portland in Oregon and between Olympia and Everett in Washington — are likely to experience the damaging effects of liquefaction.
Areas on the steep slopes of mountain ranges in Washington and Oregon are susceptible to landslides and rock falls. Landslides can cause damage to critical infrastructure, residential and commercial structures. They can also isolate communities when landslides and rockfalls cross roadways or knock out power or communications lines. Shaking from earthquakes and aftershocks often trigger many landslides and rocksfalls. The risk of landslides and liquefaction can increase when heavy rainfall causes soil to become waterlogged and saturated.
The evidence for past subduction zone earthquakes of magnitude 9.0 suggests that they recur, on average, every 500 years, but the actual intervals between events are far from predictable—such earthquakes have been separated by as many as 1,000 years and as few as 200. The estimates of the sizes of pre-1700 earthquakes are also uncertain. Cascadia has now been building up strain for over 300 years, so the next great earthquake could happen at any time.
Should the earthquake and tsunami happen tomorrow, it could affect millions of people’s lives, property, infrastructure, and environment. The number of deaths could exceed 10,000, and more than 30,000 people could be injured. The economic impacts could also be significant. For Washington and Oregon, the direct economic losses have been estimated at upward of $81 billion. These social and economic impacts could distress the region for years to come.
While the timing cannot be forecast very precisely, great subduction zone earthquakes are inevitable—they are a fundamental consequence of plate tectonics. Whether this type of earthquake is considered alone or in combination with other earthquake sources, the odds that a large, damaging earthquake will occur in the near future in the Cascadia region are very high. The more steps our communities take now to prepare, the more resilient we will be